1. Field of the Invention
The present invention relates to a substrate for a liquid jet recording head for performing recording with the recording liquid ejected from the discharging ports thereof by the utilization of thermal energy, a manufacturing method therefor, and a liquid jet recording head and a liquid recording apparatus using such a substrate. More particularly, the invention relates to a substrate for a liquid jet recording head with a supporting member and each layer which have been improved, a manufacturing method therefor, a liquid jet recording head, and a liquid jet recording apparatus.
2. Related Background Art
The liquid jet recording method, wherein recordings are performed by utilizing thermal energy to cause ink or other liquid droplets to be ejected and to fly onto a recording medium (paper in most cases), is a recording method of a non-impact type. Therefore, it has the advantages among others that there is less noise in operating it, direct recordings are possible on an ordinary sheet, and color image recordings are also possible with ease by the use of multiple color ink. Furthermore, the recording apparatus can be built with a simple structure to make it easier to fabricate a highly precise multi-nozzles. There is thus an advantage that with this type of recording apparatus, it is possible to obtain with ease recordings with a high resolution at high speeds. The liquid jet recording apparatus has, therefore, come rapidly into wide use recent years.
FIG. 9A is a perspective and broken view showing the principal part of a liquid jet recording head used for this liquid jet recording method. FIG. 9B is a vertically sectional view showing the principal part of this liquid jet recording head on a plane parallel to its liquid passage. As shown in FIGS. 9A and 9B, this liquid jet recording head is generally structured with a number of fine discharging ports 7 for ejecting ink or other liquid for recording; passages 6 provided respectively for each of the discharging ports 7 and conductively connected with each of the discharging ports 7; a liquid chamber 10 provided commonly for each of the liquid passages 6 to supply the recording liquid for the respective passages 6; a liquid supply inlet 9 arranged on the ceiling portion of the liquid chamber 10 for supplying liquid to the liquid chamber 10; and a substrate 8 for the liquid jet recording head having exothermic resistive elements 2a for each of the liquid passages 6 for giving thermal energy to recording liquid. The liquid passages 6, the discharging ports 7, the liquid supply inlet 9, and the liquid chamber 10 are integrally formed with the ceiling plate 5.
As shown in FIG. 9B, the substrate 8 for the liquid jet recording head is of such a structure that on its supporting member 1 an exothermic resistive layer 2 made of a material having a volume resistivity of a certain amplitude and then, on the exothermic resistive layer 2, an electrode layer 3 made of a material having a desirable electric conductivity is laminated. The electrode layer 3 has the same configuration as the exothermic resistive layer 2, but it has a partial cut-off portion where the exothermic resistive layer 2 is exposed. This portion becomes an exothermic resistive element 2a, that is, the portion where heat is generated. The electrode layer 3 becomes two electrodes 3a and 3b with the exothermic resistive element 2a therebetween, and a voltage is applied across these electrodes 3a and 3b to enable an electric current to flow in the exothermic resistive element 2a to generate heat. The exothermic resistive element 2a is formed on the substrate 8 for the liquid jet recording head to be positioned at the bottom of each of the liquid passages 6 corresponding to the ceiling plate 5. Further, on the substrate 8 for the liquid jet recording head, a protective layer 4 is provided for covering the electrodes 3a and 3b, and the exothermic resistive elements 2a. This protective layer 4 is provided for the purpose to protect the exothermic resistive elements 2a and electrodes 3a and 3b from the electrolytic corrosion and electrical insulation breakage due to its contact with recording liquid or the permeation of the recording liquid. It is a general practice that the protective layer 4 is formed using SiO.sub.2. Further, on the protective layer 4, an anti-cavitation layer (not shown) is provided. As a formation method for the protective layer 4, various vacuum film formation methods, such as plasma CVD, sputtering, or bias sputtering, are employed.
As the supporting member 1 for the substrate 8 for the liquid jet recording head, while it is possible to use a plate made of silicon, glass, ceramic, or the like, the silicon plate is most often used for the reasons given below.
When a glass plate is used for the supporting member 1 to produce a liquid jet recording head, heat tends to be accumulated in the supporting member 1 if the driving frequency of the exothermic resistive element 2a is increased because glass is inferior in heat conductivity. As a result, the recording liquid in the liquid jet recording head is unintentionally heated to develop bubbles, often leading to the undesirable ejection of the recording liquid and other defectives.
On the other hand, when ceramic is used for the supporting member 1, alumina is mainly employed because alumina can be produced in a comparatively large size and has a heat conductivity better than glass. Nevertheless, in a case of ceramic, it is a general practice that the powdered material is baked to produce the supporting member 1, which often results in pin holes or small projections of several .mu.m to several ten .mu.m or other surface defectives. Due to such surface defectives, short and open circuits of the wirings and other troubles may take place to cause the reduction of the yield. Also, the surface roughness is usually R.sub.a (average roughness along the center line)=approximately 0.15 .mu.m. There are thus many cases where it is difficult to obtain the surface roughness best suited for the film formation of the exothermic resistive layer 2 and others with a desirable durability. For example, if alumina is used for the production of the liquid jet recording head, there occur the peeling of the exothermic resistive layer 2 from the substrate 8 for the liquid jet recording head, and others; hence shortening the life of the durability of the recording head.
In this respect, there is a method to improve the contacting capability of the exothermic resistive layer 2 by smoothing the roughness of the surface of the supporting member 1 with a polish machining given thereto. However, since the hardness of alumina is high, there is automatically a limit for the adjustment of the surface roughness for the purpose. To counteract this, it may be conceivable that a glazed layer (a welded glass layer) is provided for the surface of an alumina supported member to produce a glazed alumina supporting member; thus solving the problem of the surface defectives and surface roughness attributable to the pin holes or small projections with the provision of the grazed layer. There is still a problem that the glazed layer cannot be made thinner than 40 to 50 .mu.m in view of its manufacturing method. As a result, heat tends to be accumulated as in the case of using glass.
In contrast to the use of the glass or ceramic for the supporting member 1, there is an advantage in using silicon for the supporting member 1 that the problems mentioned above will not be encountered. Particularly, if a polycrystalline silicon substrate is used for the supporting member 1, there is no need for any process to pickup crystals as in a case of the application of a mono crystal silicon for use. Therefore, its manufacturable size is not confined. Here, the inventor hereof et al. find that not only there is an advantage in its manufacturing cost, but also it is possible to obtain a square column ingot if the polycrystalline silicon substrate is produced by the application of a casting method. It is thus regarded as advantageously applicable from the viewpoint of the material yield when square supporting members 1 are cut for the intended use.
When silicon is used for the supporting member 1, it is a general practice that for the purpose to obtain better characteristics as the substrate 8 for the liquid jet recording head, a lower layer made of SiO.sub.2 serving as a heat storage layer is provided for the entire surface or a part of the surface of the supporting member so as to balance the heat radiating and accumulating capabilities of the supporting member 1.
Also, if the supporting member is an electric conductor, the above-mentioned lower layer should be arranged to serve dually as an insulator in order to avoid any short circuit electrically. This is convenient from the viewpoint of both design and cost. Then, as the method to form this lower layer (hereinafter referred to as heat storage layer), there are those to form it by means of thermal oxidation given to the surface of the supporting member 1 made of silicon and to deposit SiO.sub.2 on the supporting member 1 by various vacuum film formation methods (sputtering, bias sputtering, thermal CVD, plasma CVD, and ion beam, for example).
Also, depending on the structures of the substrate for the liquid jet recording head, two layers of wirings are provided in matrix on the supporting member. In this case, the wirings connected directly to this exothermic resistive layer will be provided on a wiring layer which is positioned farther away from the supporting member due to its positional relationship with the liquid passages. Consequently, the wiring layer which is closer to the supporting layer is in a mode that such a layer is buried in the heat storage layer. FIG. 12 is a schematic cross section representing the structure of the substrate for the liquid jet recording head.
For the substrate for the liquid jet recording head shown in FIG. 12, a heat storage layer 402 is formed separately for a first heat storage layer 402a and a second heat storage layer 402b. On the silicon supporting member 401, the first heat storage layer 402a made of SiO.sub.2 is provided. On the first heat storage layer 402a, a lower wiring 403 serving as a first layer for the wiring layer is formed. This first heat storage layer 402a can be formed by the thermal oxidation given to the silicon supporting member 401. The lower wiring 403 is generally made of aluminum, and is provided for driving the exothermic portions in matrix, for example. On the other hand, the second heat storage layer 402b is formed on the upper face of the first heat storage layer 402a with the lower wiring 403 thus formed so that this layer covers the lower wiring 403. The second heat storage layer 402b is formed with SiO.sub.2. Further, on the second heat storage layer 402b, an exothermic resistive layer 404, an electrode layer 405 which serves as a second layer for the wiring layer, a protective layer 406 made of SiO.sub.2, and an anti-cavitation layer 407 are provided in the same manner as the substrate for the liquid jet recording head shown in FIG. 9. The second heat storage layer 402 cannot be formed by means of the thermal oxidation due to the presence of the lower wiring 403. Therefore, it is formed by the application of the plasma CVD, sputtering, bias sputtering, or the like as in the case of the protective layer 406.
As described above, the silicon dioxide layer represented by the SiO.sub.2 layer is used for the heat storage layer and protective layer in fabricating the substrate for the liquid jet recording head. These layers are classified into (1) the layer which can be formed by means of the thermal oxidation given to the supporting member made of silicon (the heat storage layer in FIG. 9 and the first heat storage layer 402a in FIG. 12) and (2) the layer which cannot be formed by means of the thermal oxidation (the protective layer 4 in FIG. 9, the second heat storage layer 402b and the protective layer 406 in FIG. 12, or in such a case where the supporting member is made of metal or the like) or the layer which is formed with a nitride film or films other than the dioxide film. Here, according to this classification, the problems existing in forming these layers will be discussed.
(1) The layer which can be formed by means of the thermal oxidation:
For the layers formable by means of the thermal oxidation, it is desirable to conduct their formation by the thermal oxidation in view of cost and the film quality of the layer obtainable. In other words, when the layer is formed by means of those conventional vacuum film formation methods, the film thickness tends to be uneven and the film formation speed is slow as described later. Also, dust particles are easily generated at the time of film formation. The dust particles mixedly contained in the film result in the granular defectives of several .mu.m diameter. Thus, there is a possibility that this will cause breakage due to cavitation. Further, there is a problem that electric current leaks from these granular defectives to cause the electric short circuit. It may also be possible to use a spin-on-glass method or a dip-pull method to form the layer made of SiO.sub.2 on the surface of the supporting member without the application of the thermal oxidation process. However, the film quality obtainable by the application of any one of these methods is not desirable, and in order to secure a desirable film quality, it becomes necessary to conduct a heat treatment at high temperature or impure particles tend to be mixed in the film. In addition, there is a problem that in some cases, the SiO.sub.2 layer of approximately 3 .mu.m film thickness, which is required for the heat storage layer, cannot be formed.
Now, the description will be made of the characteristics of the SiO.sub.2 layer formed by means of the thermal oxidation hereunder.
The silicon substrate (supporting member) which is an object to be formed here by the thermal oxidation is a polycrystalline silicon supporting member as described above. In this respect, it has been found by the inventor hereof et al that when an SiO.sub.2 layer is formed by means of the thermal oxidation given to the surface of the polycrystalline silicon supporting member, there occurs a difference in level of approximately less than several hundred nm on the surface of the SiO.sub.2 layer due to the difference in the thermal oxidation velocities attributable to the different crystalline orientations. If such a difference in level occurs on the surface, possible damages are concentrated on that staged portion whether due to thermal shock given by heating and cooling or to the cavitation generated at the time of ejecting liquid for recording. Therefore, if the exothermic resistive elements should be formed where such a difference in level exists, there would be encountered a problem that its reliability is significantly reduced. More specifically, when the ejection of the liquid is repeated for recording, the cavitation will be concentrated on the difference in level on the surface. Thus, a problem arises that a breakage may take place earlier. In order to avoid such a problem as this, it is conceivable that the thermally oxidized surface is flattened by a polish machining. However, with an ordinary machining technique, it is impracticable to flatten a layer of less than several .mu.m thick. It is also conceivable that an extremely thick thermal oxidation layer is formed and is removed by a polish machining for the purpose. With its cost in view, this is quite disadvantageous.
(2) The layer which cannot be formed by means of the thermal oxidation:
When formation is impossible by the application of the thermal oxidation, the SiO.sub.2 layer will be formed inevitably by the application of the plasma CVD, sputtering, bias sputtering, or other vacuum film formation methods. In this case, the SiO.sub.2 layer is formed on-the wiring layer, exothermic resistive layer, and polycrystalline silicon thermal oxidation layer. This layer must be formed desirably even at a place where the difference in level exists. Also, there are some cases where a wiring layer and exothermic resistive layer are to be formed on this layer of SiO.sub.2 thus formed, it is desirable to flatten the upper surface of this layer even in the portion where the difference in level takes place. Hereunder, the description will be made of the problems existing in forming the SiO.sub.2 layer by the application of the plasma CVD, sputtering, and bias sputtering, respectively.
In the plasma CVD, the configuration of the film becomes acutely steep configuration of the wirings where difference in level takes place; thus making the film quality degraded in such portion thereof. There is also a problem that minute irregularities are created on the surface of the film to be formed. At first, the description will be made of the acutely steep configuration in the portion where difference in level exists.
FIG. 13A is a cross-sectional view showing the composition of the difference in level taking place in the SiO.sub.2 film 410 formed by a plasma CVD on an aluminum wiring 409. When the difference in level is composed in applying the plasma CVD, the cut created by the difference in level becomes deep as the portion which is indicated by an arrow A in FIG. 13A. Therefore, as shown in FIG. 13B, if a thin film 411 is formed by deposition, sputtering, or other method on the SiO.sub.2 film 410, the expansion of the film over the portion A is not good enough; thus making it thinner in that portion than the film over the flat portion. Thus, when wiring and others are formed there, the current density becomes greater to cause heat generation or wire breakage. Also, when a patterning is conducted for the wirings to be formed on the SiO.sub.2 film 410, resist is not desirably removed by the application of the ordinary photolithography technique in the portion where the difference in level occurs, and there tends to occur short circuits between the wirings. FIG. 13C is a view showing the portion represented in FIG. 13B, which is observed in the direction indicated by an arrow C in FIG. 13A. It shows the state where a film 411 (the slashed portion in FIG. 13C), an aluminum wiring, for example, on the SiO.sub.2 film 410, is extended along the differences in level. This problem arises more easily for a film between layers, that is, an SiO.sub.2 layer which is placed between a plurality of wiring layers.
When the SiO.sub.2 film is formed by the application of the plasma CVD, the film quality in the portion where the difference in level takes place becomes more degraded as shown at B in FIG. 13A. If the SiO.sub.2 film thus formed is etched with a hydrofluoric acid etching solution, the film at B is etched instantaneously because its minuteness is low whereas the film on the flat portion is being etched at a velocity two to four times that of the SiO.sub.2 film formation by the thermal oxidation. In such a portion of the film as having a low minuteness, cracks tend to occur due to the thermal stress created by the repeated heating and cooling of the heaters (exothermic portions). Therefore, when the film is used as a protective layer, its function will easily be lost. Also, for the patterning of a film which must be laminated on the SiO.sub.2 film, that is, the HfB.sub.2 film to be used for the exothermic resistive layer and the Ta film to be used for the anti-cavitation layer, for example, it becomes impossible to use any hydrofluoric acid etching solution.
Now, the description will be made of the minute irregularities on the surface of the SiO.sub.2 film which is formed by the application of the plasma CVD.
In general, there tend to occur minute irregularities on the surface of the film produced by the plasma CVD even if it is formed on a flat substrate. These irregularities on the SiO.sub.2 film will also remain on the anti-cavitation layer which is directly in contact with ink. Therefore, when the ink bubbling takes place on the heater surface, the initiation points of bubbling (bubbling nuclei) are scattered on the heater surface. Thus, the film boiling phenomenon can hardly be reproduced with stability and there is a possibility that this instability will produce adverse effects on the ejection performance.
In the sputtering method, the configuration of a film is acutely steep in the wiring portion where the difference in level takes place. The film quality of the film thus formed is not desirable. Also, there is a problem that the so-called particles are great. The fact that the configuration of the film is acutely steep in the portion where the difference in level occurs is the same as in the case of the application of the plasma CVD. Therefore, the description thereof will be omitted. Here, the film quality will be described at first.
When the SiO.sub.2 film is formed by means of an ordinary sputtering method (that is, a method to sputter an SiO.sub.2 target with Ar gas), it is impossible to form any minute film unless the substrate temperature is raised to approximately 300.degree. C. However, if the temperature is raised to approximately 300.degree. C., great hillocks are developed in the aluminum layer to be used for wirings. Particularly, when a hillock is developed at the edge portion of the aluminum wiring 409 as shown in FIG. 14, the substantial difference in the film thickness of the SiO.sub.2 film 410 formed thereon becomes great; hence degrading the covering capability as a film. In other words, cracks tend to occur at the stepping portion, and if ink is in contact with the electrodes from such cracked portions, electrolytic corrosion will ensue, also, the film quality in the portion where the difference in level occurs cannot be improved even if the substrate temperature is raised to 300.degree. C. There will be encountered the same problem as in the case of the film formed by the application of the plasma CVD.
As a method to form a film at low temperatures without degrading the film quality, it is possible to conduct sputtering an SiO.sub.2 target in an atmosphere of Ar and H.sub.2. However, it is still impossible to improve the film quality in the portion where the difference in level takes place. Also, the film configuration in such portion is the same as at B in FIG. 13A. The same problem as in the case of the film formation by the application of the plasma CVD is encountered. Moreover, if an H.sub.2 gas is added, the film formation velocity is lowered (conceivably, the more H.sub.2 is added, the lower becomes the velocity); thus reducing the processing capability.
Also, in the film formation chamber of a sputtering apparatus, a target, shield plate, shutter plate, and others are arranged to make its structure more complicated than the reaction chamber of a plasma CVD apparatus. Then, when an SiO.sub.2 and other insulation films are formed, spark discharge is generated due to charge up or the like. Thus, a problem is encountered here that the scattered materials due to the spark discharge and the deposited dust particles which cannot be removed by maintenance (cleaning) in the complicated film formation chamber fall down as particles onto the substrate and are accumulated thereon. In other words, if these dust particles are contained in the film, granular defectives of several .mu.m will ensue, and if the exothermic resistive elements are formed on the portions having such defectives, there is a possibility that the cavitation breakage occurs at the time of ejection. If the substrate is electrically conductive, electric current will leak from such granular defective portions to cause electric short circuit. Because of this, it becomes difficult to enhance the reliability and durability of a recording head to be manufactured.
The bias sputtering method is a method to flatten the configuration at the position where the difference in level takes place by applying a high frequency power also to the substrate side to utilize the sputtering effects produced by its self bias. Therefore, unlike the sputtering or the plasma CVD, there is no problem as far as the insufficient flattening of the stepping portion is concerned. FIG. 15 is a schematic view showing the composition of the stepping portion (the portion where the difference in level exists) when the SiO.sub.2 layer 410 is formed on an aluminum wiring 409 by the application of a bias sputtering method. From FIG. 15, it is clear that compared to the plasma CVD or the like, the stepping portion has been flattened. Nevertheless, as is the case of the ordinary sputtering method, particles are easily generated. Also, there is a problem that the film formation velocity is low. Here, the film formation velocity in the bias sputtering method will be discussed.
In the bias sputtering method, etching is conducted simultaneously while a high frequency bias is given to the substrate side. As a result, compared to the ordinary sputtering, the film formation velocity of the bias sputtering is reduced by an amount equivalent to the etching thus conducted. In order to make the film quality at the stepping portion and coverage desirable, there is a need for the addition of etching for more than 10% of the film formation velocity. Accordingly, compared to the ordinary sputtering, the film formation velocity is lowered more than 10%. Hence, the productivity is reduced that much. In this respect, if the bias is applied too much, the substantial film formation velocity is further lowered. Also a problem may arise that the stepping portion cannot be covered. Therefore, it is desirable to define the etching velocity to be 5% to 50% of the film formation velocity without any bias being applied.
Furthermore, both in the sputtering and bias sputtering methods, if the high frequency power applied to the cathode (target) is increased too great, the target is cracked or abnormal discharge is generated. With the technique currently available, therefore, it is considered that the film formation velocity is limited to 200 nm/min. From this point of view, these are regarded as methods having a low productivity.
As described above, when the heat storage layer protective layer, or insulation film between the wirings are formed for the substrate for the liquid jet recording head, there are many aspects which must be improved with respect to the film quality and the surface smoothness or the film formation velocity among others.